High frequency transformer



April 19, 1932. J. K. JOHNSON 1,855,055

HIGH FREQUENCY TRANSFORMER Original Filed Feb. .15, 1931 2 Sheets-SheetA l ATTORNEYS Apri 19, 1932. J. K JOHNSON 1,855,955

HIGH FREQUENCY TRANSFORMER Original Filed Feb. 13, 1951 2 Sheets-Sheet 2[55 /r/egacuy /f//Oycles /aer Serena? ATTnpN :vc

1. 515,560,1i1ed Feb. 13,1931.

v 50 fective coupling between the coils is reduced latented Apr. 19, 1932 UNITED STATES Parrsur OFFICE JonN 'KELLY JoHNsoN, or nAarsDALE, NEWYoan, assrGNoa ro HAzELrnm conf rona'rroN, A coarom'rrou or DELAWAREHIGH FREQUENCY TRANSFOBMEB Original application iiled February 1d, 1931,Serial No. 515,560. Divided and this application led y'February 28,1931.

for use inl amplifiers of medium-high fre lo-quencies, such as .theintermediate-fre uency1 amplifier of a superheterodyne typeo radioreceiver.

This application is a division of my conding original application,Serial .No.

The princi al object of this invention is to adjust the egree ofcoupling between two circmts of a carrier-frequency coupling sys'- temto the pro r value to rovide a high degreeof ampli cation, uni ormtransmission and good selectivity.

Another object is to provide a coupling system which shall be compact,inexpensive and easily adjustable? z5 l The, coupling systemscontemplated in this invention comprise an input circuit which isconnected to a pair of input terminals Vand an out ut .circuit which 1sconnected to a pair o output terminals. The input and output circuitseach include an inductance tuned by` a capacity; these inductanceelementsare coils ,which are spaced in close relat-ion to4 each other sothat there exists a vsubstantial degree 'of magnetic couplingtherebetween. The coupling system will generally be connected betweenthe output of one tube of a high-frequency amplifier and the input of asucceeding tube. There may be two or more-of these coupling systemscoupled in tandem. f y

-An important feature of the invention is the provision of an electricalconducting shielding arrangement surrounding, or partially surrounding,both coils. This shielding arrangement has the eect of reducing thedegree of coupling between the inductances of the input and outputcircuits to a lower value than would exist in the absence of the shield.The amount by .whicli the ef- Serial No. 518,986.

by virtue of the shield, depends upon the form and separation of thecoils and upon the spacing of the shield from the coils.

ln high-frequency transformers heretofore employed which compriseprimaryand secondary windings each of which is tuned to the samefrequency by fixed or adjustable capacities, it has been foundnecessary, where Y relatively loose coupling between circuits isrequired, to physically space the coils a considerabledistance apart, orin the alternative, to provide an auxiliary element between the windingsto reduce the electromagnetic coupling. If this relatively great spacingbetween the coils is not provided, or if there is present no' auxiliarymeans for reducing thecoupling, there results the well-known effect ofdouble resonance which makes it impossi- 'ble to tune the system to asingle frequency.

Thereactionofonecirc ituponthe other produces the two resonant peaks ofwhich differs from the frequencyto which each coil is individuallytuned. When the electromagnetic coupling between the two circuits isdecreased, the frequency difference between theseresonant peaksdecreases, so as opti'- mum coupling is approached, the double resonantpeaks merge into the single resonance.

It has been found possible to vary the effective coeicient oelectromagnetic coupling and hence the mutual inductance between thecoils within wide limits by vary-- ing the relative sizes of theshielding ring and of the coils and the spacing between coils. Byarranging the shielding ring close tothe coils it is possible to locatethe coils in close proximity to each other while at the same timemaintaining absolute control over the leffective coefficient of couplingand the mutual inductance betweenv coils, and hence the 9o reaction ofone circuit upon the other. Practically, it is possible to sb arrangethe elements of the coil structure that it is small, compact andinexpensive to manufacture.

Of the drawings: l

Fig. 1 illustrates a radio receiver of the superheterodyne type in whichthe intermediate-frequency amplifier employs coupling systems of thetype of this invention; Y

Fig. 2 illustrates the construction ofthe in- 10 eol ductances and thearrangement ofthe shielding`of the coupling systems;

ig. 3 shows graphically how the ratio of the coil separation to the coildia-meter affects the effective coefiicient of coupling between thecoils;

y Fig. \4 shows graphically the change in effective electromagneticcoupling as a function ofthe ratio of the coil diameter to the diameterof the shielding ring;

5 shows transmission characteristics of the coupling system with andWithout thel shield.

Fig. 1 illustrates a conventional type of superheterodyne radioreceiver. embodying the invention. The receiver comprises an ani tennacircuit 10 connected to a radio-frequency amplifier shown in generalizedform as the rectangular 11. The output of the radio-frequency amplifieris associated with a local oscillator indicated in generalized form bythe rectangle 12. The local oscillator and the radio-frequency amplifierare connected in the input of a vacuum tube modulator 13, theA purposeof the modulator being .to modulate the amplified signals from theamplifier with the oscillationsfrom the local oscillator. Theconstruction of a local oscillator and of an amplifier for this purposeand the manner of their connection in the input circuit of the modulatorare well understood in the art, and require no further `discussion here.

The modulator tube 13 is of the four-electrode type comprising an anode14, a cath.- ode 15, a control grid 16 and avscreen grid 17. Themodulator 13 operates in the well understood manner upon the signalvoltage and the local oscillatorvoltage to' produce a signal in itsoutput having a carrier frequency which is the difference between thefrequency of the radio signal and the frequency of the local oscillator.Since this difference in frequency is lower than the frequencies of theradio signaling range, itis called an intermediate frequency.

Thegoutput of the modulator is coupled to the input of'anintermediate-frequency amplifier 18 through a coupling system 19. The'amplifier 18 is also of the four-'electrode type and includes an anode20,- a cathode 21, a control grid 22 and ascreen grid 23.. The output ofthe intermediate-frequency amplifier is coupled to a detector tube 24vthrough loudspeaker 31.

a coupling system 25. The detector tube, which is also of thefour-dt\\'o `le type, includes'an anode 26, a cathode 27,`a control grid28 and a screen grid 29. The output of the detector feeds into anaudio-frequency amplifier represented by the rectangle 30. The output ofthe audio amplifier operates a There are illustrated no seni-ces ofoperating potentials for the various electrodes of the vacuum tubes ofthe receiver. The manner of applying these energizing potentials is wellunderstoodin the art and since it constitutes no part of the presentinvention, these sources are not illustrated.

The feature of this invention resides in the coupling systems 19 and 25associated with the intermediate-frequency amplifier. The couplingsystem 19 comprises two circuits, the first of which is connected in theoutput of modulator 13 and the second of which is connected in the inputof amplifier 18. The first circuit includes an inductance 32 shunted bya fixed capacity 33, connected in the anode circuit of modulator 13. Thesecond circuit includes an inductance-34 shunted by a .greater detail.

The coupling system 25 'in the output of the intermediate-frequencyamplifier 18 is similar in its circuit arrangement to coupling system19; it includes inductively related inductances 37 and 38 situatedrespectively in the output circuit of the amplifier '.18 and inthe inputcircuit of the detector 24. The inductance 37 is shunted by a fixedcondenser 39, and inductance 3S is shunted by fixed condenser 40. Y f IFig. 2 illustrates in cross-section the construction and assembly of theinductively related inductances and the associated shielding ring, ofcoupling systems 19 and 25. The arrangement comprises coils 41 and 42random wound, or layer Wound, respectively on bobbins 43 and 44. Thebobbins are fastened over a core 45 which is adapted to be fastened to abase, or chassis, by brackets 46 and 47. Coil 41 is the `anode circuitcoil and coil 42 is the grid circuit coil. The coil terminals marked A,B, C and D, represent respectively the connections to the anode, the B-battery, the cathode and the grid of the associated tubes. Surroundingthe bobbins containing the coils is a cylindrically-shaped shieldingring 48 closed at .the upper end and adapted to be fastened to the baseat the other end. The shield-ing ring is preferably heavy copper oraluminum or other metal having low specific electrical resistance; Anyjoints used in completing the ring should be of very low resistance, sothat the total electrical resistance of the entire ring is very low. The

Yshielding ring is located in close proirimity s to the coils so thatthere exists a substantial coupling between the ring and the coils.Signal current flowing through one of the coils produces a magnetic eldwhich interlinks. with the turns of the second coil and with the shield.Currents are thereby induced in the second coil and in the shield. Thecurrent induced in the shield, in turn, produces a magnetic field whichis approximately oppositein phase to the field of the first coil. As aresult, the Idegree of coupling between the two coils is reduced to alower value than would exist in the absence of the shield.'

Due to the close.l spacing between the shielding device andthe coils andbecause of the low resistance of the shielding device, the coefficientof coupling between the coils and the shield will usually be greaterthan that 4between the two coils. It has been found possible, in fact,to reduce the effective .magnetic coupling between coils practically tozero. v

The dimensions of the` coils will, of course, be'idependent upon theintermediate frequency'which the amplifier is required to transmit. Thefollowing table gives a suitable design for a coupling system fortransmitting a carrier frequency of 175 kilocycles per second, and theassociated sidebands, this frequency having been found highlysatisfactory for the intermediate frequency of a superheterodynereceiver:

Diameter of core-1/2.

Each winding-900 turns, #38 B. & S. gauge. double silk covered fcopperwire.

Vidth" of each coil-l.

Distance between coils-fig.

Outside diameter of each coil- Q.

, Inside diameter of shieldingring--1%.

Length of shielding ring-1%.

The above dimensions will provide a coil structure having an inductanceof about 8.8 millihenries in each` coil; the effective coefficient ofcoupling between each coil will be about four percent. In order to cause.each o the coils to be resonant at a frequency of 175 ilocycles, thefixed capacity shuntmlcro-farads. Fig. 3 illustrates a family of curveswhich indicate-the relationship of the effective coefficient'of couplingbetween coils to the ratio g of the separation between coils totheoutside diameter of the coils.. In this figure curve' a illustrates thevariation of the effective coefiicientof coupling between coils in theabsence of any shieldi-ng ring. Curve b illustrates the variation in thecoefficient of coupling when the coils are placed 'within a shieldingring which is so proportioned that the inside diameterl of t e ring isone and 4one-third times the'outside coil diameter. Curve c indicatesthe variation in 6 vthe 'effective coefficient of coupling when the.having the design ing -each coil should be about 100 micro`yamplification characteristic when the shieldmij inside diameter of theshielding ring is two times the outside coil diameter. A casualinspection of the curves of Fig. 3 shows that the effective couplingbetween the coils bef comes smaller when the distance between coilsbecomes greater relative to the outside coil diameter.

Fig. 4 illustrates another family of curves which may be derived fromthe curves of Fig. 3 and which show the effect" of the diameter 75 ofthe shielding ring upon the effective electromagnetic coupling betweenthe coils. ln this figure the effective coefficient of coupling isplotted against the ratio of the inside diameter of the shielding ringto thefoutside 8o coil diameter. Curve a shows the variation of theeffective coefficient of coupling between the coils when the distancebetween the coil centers is one-half the outside coil diameter. Curve billustrates the varia- S5 tion inthe effective coefficient of couplingwhen the distance between the coil centers is three quarters of theoutside coil diameter. Curve c indicates the variation when the distancebetween coil centers is equal to the outside coil diameter. f The curvesof Figs. 3 and 4; are experimental curves obtained from the coilstructures constants given `in the above table; 95

Experiments with coil and shielding ring structures of various sizes andshapes have indicated that the most important variables are the ratio ofthe-inside diameter of the shielding ring to the outside coil diameter,and the ratio of the coil separation to the outside coil diameter.

` The most .favorable ratios of theQ inside shielding ring diameter rtothe outside coil diameter lie between one and two. The spacing requiredbetween coils is spbject'to 'variation', dependent upon the desiredcoefficient of coupling and hence the mutual inductance `required, thepower'factor of the coils. the form factor of the coils, and thediameter of the shielding ring. When the coil structure is proportionedin accordance with the above table, favorable results are obtained whenthe spacing between the centers of the coils does not exceed the meanYdiameter of either coil. Where the radius of the core itself is equal toorgreater than the depth of winding of the coil'` the spacing betweencenters of the respective coils may be reduced.

to a distance which' is ofthe same order off-'31225 magnitude, asone-half of the mean diameter of the coil. lt should be understood thatthe above mentioned preferred proportions should not be construed to belimitations upon the invention, but are given to indicate how favorableresultstmav be obtained.

Fig. 5 illus-trates amplification vcharacteristics of la coupling systemsuch-as 19 and 25 of Fig. 1.-- Curve fa of Fig. 5 illustrates the- -ingring is removed; the coupling coeilcient `in'this case is about 15%.This is the condi- By the term optimum coupling is meant that degree ofcoupling between the input and output circuits of a vcoupling systemwhich will provide the maximum amplification. It is well known that whenthe coupling between a pair of syntonously tuned circuits of a couplingsystem is optimum, or less than optimum, the system is characterized by"a'single resonance. When the coupling becomes greater than optimum, theresonances s read, giving rise to the doubleresonance e ect. Thedistance in the 'frequency scale between the two resonances is greater,the greater the degree of coupling;

so it is possible to obtain any desired spac-l ing byv adjusting thecoupling by means of the shield.

of frequenciesyother design conditions require that there be a singleresonance. It is within the contemplation of this invention to'provideeither type of resonance characteristic. It is clear that whether adouble resonance effect or'a single resonance e'ect is to be obtained,the required coeicient of coupling is quite critical. This criticalvalue can be readily obtained in accordance -with this invention, bysuitably proportioning the dimensions of the shielding ring in relationto the coil dimensions.

Although there is illustrated in Fig. 1 only oneA double tuned couplingsystem between each successive tube, there are instances where it isdesirable vtoprovide more than one such double tuned system betweenstages. A plurality of double tuned systems may be coupled eitherinductively or capacitively in tandem between successive Vacuum tubes.

Although the coupling' systems have been described as being particularlywell lsuited for use in the intermediate frequency amplifiers ofsuperheterodyne receivers, the in- -vention should not be construed tobe limited to such receivers. The coupling systems of thisr inventionare equally applicable to radiofrequency amplifiers and to .any otherhighfrequency s stem.

Y What is c aimed is: l"

VY1. "Ah'gh-frequency coupling syzem comprising two coils and ashielding device, said coils being electroma etically coupled to i eachother and having t cir centers separated by a distance which is betweenone-half and two times their outside diameters, said .shield- -ingdevice surrounding said coils, the diameter of said shielding devicebeing of the same order of magnitude as the separation between the coilswhereby lthe coupling between said coils and said device issubstantially greater than the coupling between said coils.

2. A high-frequency transformer comprising a primary coil, a secondarycoil and a shielding ring, said coils being multi-layer wound, thedistance between the centers of said coils being no greater than theoutside diameter of .said coils, and said shielding ring surroundingsaid coils, the ratio of the inside diameter of said shielding ring tothe outside diameter of said coils being no greater than r 3. Ahigh-frequency transformer compris# ing a primary-coil, a secondary coilyand a shielding device surrounding said coils, said'- shielding devicehaving an lelectrically conducting surface situated no further from saidcoils than half the coil diameter, said coils Y being spaced apart by adistance no less than the approximate radial depth of saidwindings,whereby the coeiiicient of coupling between said coils and saidring is greater than the coeicient of coupling between said coils.

4. A high-frequency transformer comprising a primary coil, a ,secondarycoil and a 'shielding ring, the coefficient of coupling between saidcoils being about 15 per cent in theI absence of said shielding ring,and said shielding ring closely surrounding said coils by a distancesuicient'to reduce the effective coecient 'of coupling between saidcoils to about 4'per cent. l

5. -A high-frequency transformer compris- -ing a pair of coils, each ofsaid' coils being wound in layers, said coils being co-axially situatedand separated from each other by av diameter of said coils, said' shieldbeing placed closely around' said coils so that the coeicient ofcoupling Abetween one of said coils and said shield is greater than thecoeilicient of coupling between said coils would be in the absence 'ofsaid shield.

7. A shielded' high-frequency transformer according'to claim 6 in whichsaid coils are wound with approximately the same number ofturns.

8. A carrier frequency transformer adapted to transmit a carrier waveand side bands corresponding to voice waves comprising a pair of coilsplaced in such close inductive relation to each other that when saidcoils are acting alone, transmission of signals through them ischaracterized by `a pair of resonance peaks, a low resistance shieldingmeans surroundinv said coils and having a diameter less than twice thatof said coils whereby said resonant peaks become more closel spaced andare separated by a frequency iference no greater than the frequencyrange of said side bands.

In testimony whereof I ax my signature.

J. KELLY JOHNSON.,

DIS'OLAlMr-:R

1,855,055.-Jm Kelly John son, Hartsdale, N. Y. HIGHFREQUENCYTRANSFORMER. Patent dated A prll 19, 1932. Disclaimer tiled March- 26,1937, by the asslgnee, Hazeltzne Corporation.

Hereby enters this disclaimer to claims 1 and 8 of said patent.

[Qic'ial Gazette April 20, 1.937.]

